Techniques for the positioning of mobile devices in a wireless environment are important for a number of applications, such as emergency location services, ad hoc networks, and vehicle tracking.
In the mobile phone industry, competing technologies address the specific issues of Emergency-911 (E-911) position location. Broadly speaking, these technologies fall into two categories: (1) network-based technologies and (2) Global Positioning Systems (GPS) technologies. The network-based technologies use network-based metrics (e.g., time of flight, angle of arrival etc). The GPS technologies use line-of-sight measurements to satellites.
Network-based and GPS technologies each attempt to meet the US Federal Communications Commission (FCC) requirements for E-911 Emergency Location Services. The FCC requirements set specific statistical requirements on the position accuracy on any technology. Currently the FCC's E-911 Phase II Plans dictate that 67% of all attempts for a position location must have an accuracy of less than 50 m, and 95% of calls must have an accuracy of less than 150 m (Federal Communications Commission E-911 http://www.fcc.gov/911/enhanced/). Although not mandated, it is generally accepted that acquisition time of a position fix must be in a reasonably short time scale. A positioning technology that can deliver a position fix within a timescale of 30 seconds is generally considered reasonable for E-911 services, since such a small amount of time has negligible impact on the total time taken for the emergency services to reach the emergency scene.
GPS-based techniques are known to provide accurate position measurements. However, such techniques are prone to a lengthy GPS acquisition time (the length of time taken from the time a GPS device is powered on until that device obtains a position fix). A principal reason for this is that the device does not know a priori which GPS satellites are currently in view and must undertake an exhaustive search to find the satellites in view. This is termed a “cold start”. Generally speaking in cold start mode, a GPS device can take up to several minutes to find a location fix, which is a time-scale deemed inappropriate for many applications, such as E-911.
One simple approach to overcome this difficulty involves simply keeping the GPS device switched on at all times. After an initial fix, the satellite almanac and ephemeris data are continuously kept up-to-date, and a cold start phase can be avoided. In this mode, the GPS device is said to be in “hot start” mode. A principal disadvantage of always keeping the device in a hot-start mode is the drain on the battery resources of the mobile device. Battery power in a mobile device is a precious resource, and any steps that can be taken to maximize battery life are advantageous. Simply keeping a GPS device powered on at all times to avoid the cold start is generally considered an unacceptable solution. This is particularly the case for an application like E-911, where the GPS location finding capability is likely to be only rarely utilized.
A potential resolution to the problem of cold starts can be found in the Assisted-GPS (A-GPS) system proposed by Qualcomm; see Qualcomm White Paper, How A-GPS Works, 2001 (http://www.snaptrack.com/pdf/How_aGPS_works.pdf). In this system, the network “assists” the mobile device by transferring updated almanacs and ephemeris, collected by base stations, to the mobile device upon receipt of a location request. This speeds up the GPS acquisition of a mobile device relative to the acquisition from a cold start. The system also integrates network positioning information to find optimal position accuracy. However, this approach at attempting to solve the cold start problem comes at the cost of increased complexity in the network system design and incurs additional costs for the required extra infrastructure (i.e. network hardware and communications). This system is also not workable if the mobile device is disconnected from a network, but still requires a GPS position fix.
Another system attempting to address cold start is the use of periodic power-down of the GPS device. In this mode, the device is periodically switched off for a set period. As long as the time switched off remains small, at each onset of the power-up phase, the GPS device effectively commences in hot start mode. This periodic power-down of a GPS device is termed “TricklePower” by GPS chipset manufacturers; see Laipac Technology Inc., Reference Guideline to TF GPS, September 2001, (http://www.laipac.com/datasheets/gps/TF%20GPS%20Series.pdf). In this mode, a user is allowed to set initially the power-up period, Pu. For example, setting Pu=30 minutes means the GPS device powers up every 30 minutes to acquire a position fix. Once the fix is obtained (usually within seconds), the GPS device enters power-down mode.
Simply guessing what value to set Pu at is not optimal. A need clearly exists for an improved GPS acquisition system.